One of the most important problems in control is that of controlling an uncertain system in order to have its output track a given reference signal. Previously developed control system design methods that apply to this class of uncertain systems include (1) designs based on the theory of differential games and L2-gain analysis; (2) geometric methods; (3) methods for adaptive design of observers and output feedback controllers; (4) methods for employing high gain observers; (5) backstepping algorithms; (6) input-to-state stability method; (7) methods for decentralized stabilization of interconnected systems, among others.
It is of interest to apply a systematic design approach for controlling system outputs in the presence of structured uncertainties, such as unmodeled dynamics and disturbances. While for linear systems stabilization and tracking can always be achieved by output feedback using standard design methods such as pole placement, separation principle, or linear quadratic regulation (LQR), for nonlinear systems the task is often difficult. Most of the nonlinear control methods, robust or adaptive, impose the assumption on asymptotic stability of the zero dynamics and thus are limited to minimum phase systems. It would be desirable to provide apparatuses and methods for adaptive output feedback control of non-minimum phase uncertain nonlinear systems